Metal core coiled cords have wide uses including cords of old-type telephones (whose body is connected to a receiver by means of a wire) because of stretchability and excellent capability for storage when compressed.
Optical fiber coiled cords also already exist.
Previous mainstream optical fibers are shortwave-band multimode fibers (see, 1985 Institute of Electronics, Information and Communication Engineers, National Convention, 2116, “Development on Optical Coiled Cords”, Dazai, et al., pp. 9-106; and 1985 Institute of Electronics, Information and Communication Engineers, National Convention, 2117, “Study on Prototype Optical Coiled Cords”, Kobayashi, et al., pp. 9-107).
The fabrication method for a coiled cord winds an optical fiber cord around a winding core, followed by high-temperature heat treatment for holding its coiled shape caused by heat deformation. In a quartz optical fiber, its coiled shape is held by causing heat deformation to its optical fiber coatings and cord coverings, while in a plastic fiber, its coiled shape is held by causing deformation to the optical fiber itself as well as its optical fiber coatings and cord coverings.
The problem in replacing a metal cord core with an optical fiber cord core is that there is an increase in optical transmission loss caused when the optical fiber is bent or twisted. Regarding a bend loss, there is recently reported an optical fiber whose tolerance is higher than that of conventional optical fibers.
For instance, a Holey fiber (hereinafter, “HF”) has a plurality of air holes around a core to reduce an effective refractive index of the optical fiber to thereby enhance its light confinement effect, which results in a less optical loss, even when bent with a small bend diameter, than that of conventional optical fibers. As an example, there is an HF whose optical loss is 0.001 dB even when bent with a diameter of 10 mm. For that reason, almost no optical loss increase would be seen by use of the HF as an optical fiber for optical fiber coiled cords, even when the HF is formed in a coiled cord shape consisting of a series of spiral small bends. (See, 2003 Institute of Electronics, Information and Communication Engineers, C-3-90, “A study on practical use of Holey fibers”, Yao-B, et al.)
The purpose of a coiled cord is to set and vary freely the distance between two points connected by the cord. By use of an optical fiber coiled cord using an HF, even when the HF is bent with a small diameter, no loss increase is caused, and even when the cord is stretched and compressed, no loss variation is caused, allowing the optical fiber coiled cord to be very stable in the loss property for stretching and compression.
To maintain stretchability of the coiled cord, the distance between ends of the cord is variable, but there is concern for stretching the cord to its entire elongated length (making the cord straight with no bends) when varied.
Optical connectors for mechanical connection are typically respectively attached to two ends of an optical fiber. Quartz is often used in making an optical fiber, whose diameter is very small, typically 125 μm. For that reason, when optical connectors are attached and fixed to the optical fiber, the optical fiber-holding force is not large, the optical connectors and optical fiber cord cannot be tensioned with very strong force. For the JIS C 6821 (an optical fiber mechanical property testing method), there is a tension test standard which defines that the force acting on an optical connector due to tension of a coiled cord is less than 5 kg. No problem arises with an elongation of a coiled cord whose tensile force is smaller than 5 kg defined, so that there is sufficient room for the elongation of the coiled cord.
Because an optical fiber coiled cord is, in many cases, used under the conditions for setting freely the distance between optical connectors, which can cause the coiled cord to be tensioned with a stronger force than the standard (less than 5 kg) in such a manner that its entire elongated state is repeated, however, there is concern for breakage of the optical fiber coiled cord, or for breakage of the optical connectors due to an excessive tensile load acting thereon. In this manner, when the distance between the optical connectors is varied, there is fully considered to be the possibility of an excessive force exceeding 5 kg acting between the optical fiber cord and the optical connectors.
That is, it is expected that, in an optical fiber coiled cord, there would be many cases where a tensile load is applied from the optical fiber cord to its optical connectors with more frequency than in typical optical fiber cords, and in the worst case, there is concern for breakage of the optical fiber cord and the optical connectors.
Accordingly, it is an object of the present invention to provide an optical fiber coiled cord in which no damage is caused to optical connectors and the optical fiber cord when the optical fiber cord is tensioned.